1. Field of the Invention
The present invention relates to a heat-resistant coated member for use when sintering or heat-treating metals or ceramics in a vacuum, an inert atmosphere or a reducing atmosphere.
2. Prior Art
Powder metallurgy products are generally manufactured by mixing a binder phase-forming powder with the primary alloy, then kneading the mixture, followed by pressing, sintering and post-sintering treatment. The sintering step is carried out in a vacuum or an inert gas atmosphere, and at an elevated temperature of 1,000 to 1,600° C.
In a typical cemented carbide manufacturing process, a solid solution of tungsten carbide with cobalt or of titanium carbide or tantalum carbide is comminuted and mixed, then subjected to drying and granulation to produce a granulated powder. The powder is then pressed, following which such steps as dewaxing, pre-sintering, sintering and machining are carried out to give the final cemented carbide product.
Sintering is carried out at a temperature at or above the temperature at which the cemented carbide liquid phase appears. For example, the eutectic temperature for a ternary WC—Co system is 1,298° C. The sintering temperature is generally within a range of 1,350 to 1,550° C. In the sintering step, it is important to control the atmosphere so as to enable cemented carbide correctly containing the target amount of carbon to be stably sintered.
When cemented carbide is sintered at about 1,500° C., green specimens placed on a carbon tray often react with the tray. That is, a process known as cementation occurs, in which carbon from the tray impregnates the specimen, lowering the strength of the specimen. A number of attempts have been made to avoid this type of problem, either by choosing another type of tray material or by providing on the surface of the tray a barrier layer composed of a material that does not react with the green specimen. For example, ceramic powders such as zirconia, alumina and yttrium oxide are commonly used when sintering a cemented carbide material. One way of doing so is to scatter the ceramic powder over the tray and use it as a placing powder. Another way is to mix the ceramic powder with a solvent and spray-coat the mixture onto the tray or apply it thereto as a highly viscous slurry. Yet another way is to form a coat by using a thermal spraying or other suitable process to deposit a dense ceramic film onto the tray. These techniques are described in JP-A 2000-509102. Providing such an oxide layer as a barrier layer on the surface of the tray has sometimes helped to prevent reaction of the tray with the specimen.
However, reaction with the tray arises even with the formation of such a barrier layer. As a result, after use in one or two sintering operations, the barrier layer on the tray cracks and delaminates.
Delamination of the film allows the carbon tray to react more easily with the specimen. Moreover, given the risk that the film will delaminate, fragment into small pieces, and become incorporated into the green specimen during sintering, a new tray must be used each time sintering is carried out.
A need has thus been felt, particularly in regards to the use of such trays in sintering, for a way to prevent the specimen from reacting with the barrier layer and the barrier layer from reacting with the tray and delaminating. There exists in particular a desire for a tray material which has a long service life and which, regardless of how many times the tray is used in the sintering of powder metallurgy products, does not result in reaction of the specimen with the barrier layer or in separation of the barrier layer from the tray substrate.